THIS invention relates to the compaction of soil and in particular to the compaction of soil using an impact roller.
The term "impact roller", as used initially in U.S. Pat. No. 2,909,106, refers to a compactor mass of non-round shape which, when towed over a soil surface, produces a series of periodic impact blows on the soil surface. The compactor mass of an impact roller has a series of spaced apart, salient points on its periphery. Each such salient point is followed by a re-entrant portion of the periphery and each re-entrant portion is followed in turn by a compacting face. As the impact roller is towed over the soil surface, for instance by means of a tractor, it rises up on each salient point and then fails forwardly and downwardly as it passes over that point, with the result that the following compacting face applies an impact blow to the soil surface.
The coupling between the tractor and the compactor mass is resilient in nature to allow for the necessary forward and downward falling motion undergone by the mass as it passes over each salient point.
In practice, as the compactor mass is towed over the soil surface, it produces a series of indentations in the soil surface, spaced apart in the direction of movement of the mass.
The longitudinal spacing of the indentations is the same as the peripheral spacing of the compacting faces of the compactor mass. For example a mass having three salient points and compacting faces and a total peripheral dimension of 6 m, which would be typical in current soil compaction practice, will produce a soil indentation approximately every 2 m. At a typical operating speed of 10 km/h impact blows are applied to the soil surface at a frequency of about 1,4 Hz.
The resulting cyclical shock reaction forces on the tractor can cause severe vertical bounce, pitching and rolling of the tractor and compactor mass and consequent damage to mechanical components and discomfort for the tractor operator. The depth of the soil indentations and hence the severity of the tractor bounce phenomenon is dependent upon factors such as soil density and moisture content, and energy per impact blow delivered by the compactor mass.
With a view to countering the bounce phenomenon, common practice is to make a first pass over the soil at an efficient operating speed of, say 10 km/h, and then reduce speed during subsequent passes as the indentations in the soil surface become more severe. A reduction in speed results however in a reduction in productivity and efficiency.
It is also common practice to use a motor grader to smooth the surface of the soil continuously during the compaction process. Some of the soil cut and moved by the grader blade falls into the indentations for compaction during subsequent passes of the compactor mass. In addition, water is generally sprayed into the soil surface at intervals during the compaction process in order to improve the compactibility of the soil. This watering of the soil performs a second important function in preventing the formation of a layer of surface dust which has the effect of attenuating the compaction blow.
While watering and grading can to some extent alleviate the problems of tractor bounce, the fact remains that most of the soil that is relocated by the grading operation is not used for smoothing out the indentations, due to the excessive width of the grader blade in relation to the track of the usual compactor mass. Furthermore, existing practice is to water a wide zone of soil surface with a spray bar, generally in excess of three metres wide. Thereafter the uneven surface is bladed with a grader. Most of the water used in thus preparing a large area for compaction is lost through evaporation into the atmosphere before compaction of the soil is achieved.